This invention relates to processes for making polyamide fiber and more particularly to a process for making polyamide fiber which contains additives including catalysts, stabilizers or both and the products made thereby which are particularly useful as staple for papermaking machine felt.
Stabilizers are often added to polyamides such as nylon 66, poly(hexamethylene adipamide), and nylon 6, poly(.epsilon.-caproamide), for the purpose of reducing thermal degradation and chemical attack. High levels of such stabilizers are desirable when the intended use of such fiber is in an environment with particularly harsh conditions. One such use of polyamide fiber is as staple used as in papermaking machine felts. Such felts are often exposed to highly alkaline, oxidizing aqueous solutions which can seriously shorten the service life of the felt.
There are several known methods for adding the stabilizing agents to polyamides. One method is to introduce a solution of the stabilizer into an autoclave during the polymerization step. The amount of stabilizer which can be introduced by this method is limited, however, due to the violent foaming that occurs during autoclave polymerization when stabilizers are added in solution form. A similar reaction occurs when large amounts of stabilizer solutions are added to continuous polymerizers. The normal maximum concentration in polyamides on commercial autoclaves and continuous polymerizers using this method is typically 0.05 weight %.
For fiber to be used for papermaking machine felts, it is also desirable sometimes to spin polyamides which have a high formic acid relative viscosity to improve resistance to wear from flexing, impact and abrasion. It has been demonstrated that an increase in molecular weight of a polyamide will increase the toughness, modulus of elasticity, and impact resistance. However, when the polyamide supply for such fiber is polyamide flake, it is often difficult to obtain the desired high relative viscosity while maintaining polymer quality, i.e., low level of cross-branching. While it would be desirable to increase the relative viscosity in the flake by using a high quality of catalyst in an autoclave, it has been found that difficulties similar to those encountered with stabilizers can occur when attempting to add catalysts in high quantity. In addition, high quantities of catalyst in the autoclave can cause severe injection port pluggage and complications to injection timings during autoclave cycles. High quantities of catalysts injected into continuous polymerizers place stringent demands on equipment capability because of high levels of waterloading.